1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, copying machine, or facsimile machine, and a line width correction method therefor.
2. Description of the Related Art
An electrophotographic image forming apparatus which electrostatically forms an image is known to suffer the difference between the line widths of the vertical and horizontal lines of a formed image owing to structural factors of the image forming apparatus. The vertical line means a straight line in the sub scanning direction perpendicular to the rotating shaft of a photosensitive drum. The horizontal line means a straight line parallel to the rotating shaft (i.e., main scanning direction) of the photosensitive drum.
Conventionally, the difference between the line widths of vertical and horizontal lines influences the proportion of a small character. This problem becomes more serious because recent digital printing allows free crisscross rotation of electronic data, and the direction of an image to be formed can be freely changed. That is, crisscross rotation is executed more frequently than conventional printing. Owing to the above-mentioned inherent factor, that is, the difference between the line widths of vertical and horizontal lines, the density and tint of a rotated image differ from those of an image which is not rotated.
To solve this problem, the median of the line width is adjusted by the spot diameter of a laser, the pulse width of a laser beam, or the settings of the developing device. In practice, however, the ratio of vertical and horizontal line widths often fails to be 1 due to variations in these and other conditions. To solve this, Japanese Patent Laid-Open No. 11-52636 discloses a method of measuring the size of an output image with respect to an image pattern and correcting the image in accordance with the measurement result.
However, the following problem arises when measuring the size of an output image with respect to an image pattern and correcting the image in an image forming apparatus having the above-described arrangement.
More specifically, the size of one dot at 600 dpi, which is most popularly used, is about 42 μm. To measure the line width itself for correction, a measurement sensor having a resolution of at least 10 μm or less is necessary. It is difficult in terms of cost and space and also technically difficult due to apparatus vibrations to arrange a sensor having a resolution of 10 μm or less in an actual image forming apparatus.